Pi is the ratio of a circle's circumference to its diameter. Huh?!? When clothed in mathematical mumbo-jumbo, the concept can sound confusing. John Reid's animated definition of Pi (above) says exactly the same thing, but the animation makes it so much clearer.

The animation also demonstrates Pi's unintuitive nature. The distance across the wheel is relatively easy to approximate at a glance, but it still seems incredible that the distance around it can be over 3 times the diameter! If you want a better idea of how deceptive this can be to people, you can even use this concept to win bar bets! Yes, Pi is actually useful in the real world.